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Cassini continues its extended tour of Saturn on May 17 with Rev111, the spacecraft's 112th orbit around the Ringed Planet. Cassini begins Rev111 early on May 17 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.47 million kilometers (916,000 miles) from Saturn's cloud tops. The spacecraft remains in a high-inclination orbit, providing an opportunity to study the rings and the polar regions of Saturn and its satellites.
Cassini starts its observations for Rev111 shortly after apoapse. During the first two days of Rev111, Cassini ISS will acquire two calibration observations of the bright, A-type star, Vega. On May 18, ISS also will take an astrometric observation of several of Saturn's small satellites including Prometheus, Pallene, Methone, Janus, and Epimetheus. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be effected by gravitational interactions with the larger icy moons.
Cassini encounters Titan on May 21 at 21:27 UTC for the 56th time and the second encounter this month. This flyby is also the fourth in a series of 10 flybys between April and August 2009 that will be spaced 16 Earth days, or one Titan day, apart and will occur as Cassini is inbound toward Saturn. The close approach distance is only 965 kilometers (600 miles), close to the lowest safe altitude for a Titan flyby. This flyby (known as T55) will allow imaging of the southern trailing hemisphere of Titan outbound to the encounter, similar to the area observed during the previous three encounters in this series. On approach to Titan, the Composite Infrared Spectrometer (CIRS), Ultraviolet Imaging Spectrometer (UVIS), and Visual and Infrared Mapping Spectrometer (VIMS) teams will be controlling spacecraft pointing, or will be considered "prime." CIRS will focus on measuring Titan's surface temperature over its northern polar regions in the far- and mid-infrared. VIMS and UVIS will use their time to measure the composition of Titan's atmosphere by looking at Titan's disk at near-infrared and ultraviolet wavelengths. UVIS will focus on the atmosphere's emission spectrum, looking particularly at emissions from the nitrogen and hydrogen in Titan's atmosphere. UVIS can also map simple hydrocarbon absorption spectra. ISS will acquire ride-along imaging during these observations.
During closest approach on T55, RADAR will be prime. On approach, RADAR will perform several scans using its scatterometry mode, acquiring regional backscatter images of Titan's surface to measure the roughness of Titan's surface. RADAR also will take a distant HiSAR observation of the terrain north of Xanadu as well as an altimetry swath in the same region. In the 18 minutes surrounding closest approach, RADAR will acquire a Synthetic Aperture RADAR (SAR) swath running northeast to southwest from north of Xanadu (around 33 degrees north, 127 degrees west), across Shangri-la, and ending in the south polar region near 70 degrees south, 270 degrees west. Among the features that RADAR will observe in this swath include Crete Facula (an area of bright terrain in northeast Shangri-la), interesting dune-filled terrain southwest of Shikoku Facula, and an east-west dark lineament in Titan's southern mid-latitude terrain named Hobal Virga. Following this SAR swath and Titan closest approach, RADAR will repeat the sequence from before the encounter, acquiring altimetry, or altitude measurements, near 62 degrees south, 270 degrees west and HiSAR imaging near Ontario Lacus. Finally, RADAR will take a scatterometry scan of the southern trailing hemisphere. The Ion and Neutral Mass Spectrometer (INMS) will ride-along with the RADAR SAR observations, measuring the composition of Titan's upper atmosphere.
Following close approach, CIRS will be prime for much of the time, though ISS will have a short observation during the period. ISS will take an 11-frame, full-disk mosaic from a distance of 243,000 to 272,000 kilometers (151,000 to 169,000 miles). This mosaic will cover the southern trailing hemisphere of Titan, an area Cassini has observed during the last few encounters. During CIRS's observation time, the infrared spectrometer will measure the spectra of Titan's limb hazes, measure the temperature of the moon's stratospheric hazes and the surface, and conduct several compositional integrations in order to measure methane distribution and to look for trace gas components.
Following the T55 flyby, Cassini imaging will be focused on Saturn's ring and satellite systems. Cassini will turn its cameras to Titan for an observation on May 23, occurring while the satellite is in Saturn's shadow. Unlike a similar observation in Rev110, the exposure times are not optimized for looking at secondary illumination on Titan's atmosphere from secondary light sources such as the rings, atmospheric emission, and other satellites. On May 24, ISS will observe Pan and Daphnis, two moonlets orbiting within gaps in Saturn's outer A ring, and their shadows on the A ring.
On May 25, Cassini reaches periapse, its closest point to Saturn on Rev111. At this point, Cassini will be 673,580 kilometers (418,543 miles) from Saturn's cloud tops, between the orbits of Rhea and Titan. Also on May 25, ISS will acquire a scan of the outer A ring, looking for eccentric features such as large ring particles or waves along the margins of the Encke and Keeler Gaps that might cast shadows. ISS also will take a look at Titan's sub-Saturn hemisphere from a distance of 1.31 million kilometers (817,000 miles). On May 26, Cassini will take a movie of the right ansa of the F ring as the spacecraft approaches the ring plane from below. The observation is designed to study the evolution of channels and streamers created by Prometheus when it is at apoapse in the narrow ring. These effects are predicted to become more pronounced as Prometheus dives deeper into the F ring as the apoapse of Prometheus's orbit and periapse of the F ring become aligned, which will occur in December. On May 27, ISS will observe the shadows of Pan and Mimas on Saturn's ring system and will take an astrometric observation of several of Saturn's small satellites, including Prometheus, Pallene, Methone, Daphnis (in the shadow of Saturn), and Pan.
On May 28, ISS will observe Titan's northern leading hemisphere from a distance of 1.6 million kilometers (996,000 miles). The main goal of the observation is to look for clouds in the north polar region, as has been observed during the last few orbits. ISS also will observe the shadows of Mimas and Janus falling across the Saturnian ring system. ISS will take three more astrometric observations of several of Saturn's small satellites on May 29, 30, and 31, observing Prometheus, Atlas, Epimetheus, Pandora, Methone, Telesto, and Pan. On May 30, ISS also will observe the shadows of Mimas, Pan, and Prometheus on Saturn's rings.
During the last full day of observation for Rev111, June 1, Cassini ISS again will take a look at moon shadows on Saturn's rings, including those of Pandora, Pan, and Mimas. ISS will acquire another astrometric observation of Saturn's small satellites, this time taking a look at Epimetheus, Janus, Pan, Pandora, Helene, and Calypso.
Cassini reaches apoapse on early on June 2, bringing Rev111 to an end and starting Rev112. Rev112 will include Cassini's 57th flyby of Titan, T56.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).